In recent years, there has been diversification of a thin film solar cell as a typical example of a thin film photoelectric converter, having led to development of a crystalline thin film solar cell in addition to a conventional amorphous thin film solar cell, and also to commercial application of a hybrid (a kind of stacked) thin film solar cell, in which these cells are stacked.
Normally, the thin film solar cell includes a transparent conductive film, one or more semiconductor thin film photoelectric conversion units, and a back electrode sequentially stacked on a substrate having an insulating property at least in its surface. One photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n-type layer.
The i-type layer as a substantially intrinsic semiconductor layer accounts for a large fraction of a thickness of the photoelectric conversion unit, and a photoelectric conversion effect occurs mainly inside this i-type layer. Therefore, a film thickness of the i-type layer as the photoelectric conversion layer is preferably large for light absorption, but when the i-type layer is made thicker than necessary, the time and cost for its deposition increase.
Meanwhile, the p-type and n-type conductive layers each serve to generate a diffusion potential inside the photoelectric conversion unit, and a level of this diffusion potential influences a value of an open circuit voltage, which is one of the important characteristics of the thin film solar cell. However, these conductive layers are inactive layers and thus do not contribute to photoelectric conversion, and light absorbed by impurities having been doped into the conductive layer does not contribute to power generation and is lost. It is thus preferable to make the film thicknesses of the p-type and n-type conductive layers as small as possible within the range that sufficiently generates the diffusion potential.
With respect to the photoelectric conversion units as described above, a unit with an amorphous i-type photoelectric conversion layer is called an amorphous photoelectric conversion unit and a unit with a crystalline i-type photoelectric conversion layer is called a crystalline photoelectric conversion unit, regardless of whether the p-type and n-type conductive layers included therein are amorphous or crystalline. An example of a thin film solar cell including the amorphous photoelectric conversion unit is an amorphous thin film silicon solar cell using amorphous silicon for the i-type photoelectric conversion layer. Further, an example of a thin film solar cell including the crystalline photoelectric conversion unit is a crystalline thin film silicon solar cell using microcrystalline silicon or polycrystalline silicon for the i-type photoelectric conversion layer.
Generally, in a semiconductor for use in the photoelectric conversion layer, a light absorption coefficient becomes smaller with increase in wavelength of light. Especially in the case of the photoelectric conversion material being a thin film, since sufficient light absorption does not occur in a wavelength region with a small absorption coefficient, an amount of photoelectric conversion is restricted due to the thickness of the photoelectric conversion layer. Therefore, an innovation has been made to form a light-scattering structure in which light incident inside the photoelectric converter resists escaping outside, making a practical light path length larger for sufficient absorption, and thereby generating a large photocurrent. For example, a texture transparent conductive film with uneven shaped surface has been used so that light transmission scattering occurs.
Incidentally, a thin film photoelectric converter having a large area is typically formed as an integrated-type thin film photoelectric conversion module. In other words, the integrated-type thin film photoelectric conversion module has a structure in which a plurality of photoelectric conversion cells, having been parted into small areas on a support substrate, are electrically connected in series. Each of the photoelectric conversion cells is usually formed by sequentially performing formation of a first electrode layer, one or more semiconductor thin film photoelectric conversion unit and a second electrode layer, and patterning by a laser beam.
In other words, in manufacturing an integrated-type thin film photoelectric converter, a processing technique using a laser beam has an important influence upon productivity and photoelectric conversion performance of the photoelectric converter. Generally, in this laser beam processing technique, it is easy to perform processing of parting a semiconductor photoelectric conversion layer that is apt to absorb a laser light into a plurality of regions. On the other hand, as for a metal layer that reflects a laser light or a transparent conductive layer that is apt to transmit a laser light therethrough, it is not easy to perform processing of parting each of those layers independently.
FIG. 6 illustrates a schematic sectional view of a method for producing an integrated-type thin film photoelectric converter disclosed in Patent Document 1. It is to be noted that in drawings of the present application, like reference numerals denote like or corresponding portions. Further, in the drawings of the present applications, relations of dimensions, such as lengths, widths and thicknesses, are appropriately changed for the sake of clarification as well as simplification of the drawings, and actual dimensional relations are not shown. In particular, the relation of thicknesses is appropriately changed and drawn.
In FIG. 6(a), at the first setout, a transparent tin oxide layer 2, a laser light absorption layer 3 and a back electrode layer 4 are sequentially stacked on a transparent glass substrate 1. The transparent tin oxide layer 2 can be deposited by thermal CVD method. Such a transparent tin oxide layer 2 has a textured surface structure with fine unevenness, which influences a surface structure of the back electrode layer 4, in order to improve light absorption efficiency inside the semiconductor photoelectric conversion layer by diffuse light reflection from the surface of the back electrode layer. As the laser light absorption layer 3, an amorphous silicon (a-Si) layer is deposited by a plasma CVD method. As the back electrode layer 4, an Ag layer is deposited using a magnetron sputtering device.
In FIG. 6(b), the substrate taken out of a sputtering reaction chamber is set on an X-Y table, and a plurality of parting line grooves D1 are formed by use of a laser beam LB1 incident from the transparent glass substrate 1 side so that the stack of the transparent tin oxide layer 2, the laser light absorption layer 3 and the metal back electrode layer 4 is parted into a plurality of regions. Since the laser beam LB1 is efficiently absorbed by the laser light absorption layer 3 through the transparent glass substrate 1 and the transparent tin oxide layer 2 to generate heat, the transparent tin oxide layer 2 and the back electrode layer 4 can be simultaneously subjected to parting processing with relative ease. The plurality of parting line grooves D1 as thus formed are mutually in parallel, and extending in a direction orthogonal to the surface of the figure.
In FIG. 6(c), a semiconductor photoelectric conversion layer 5 is deposited using a plasma CVD device so as to cover the parted back electrode layer 4 and the parting line grooves D1.
In FIG. 6(d), the substrate taken out of a plasma CVD reflection chamber is set on the X-Y table, and a plurality of parting line grooves D2 are formed by use of a YAG laser beam LB2 incident from the semiconductor photoelectric conversion layer 5 side thereby part the semiconductor photoelectric conversion layer 5 into a plurality of photoelectric conversion regions. Each of these parting line grooves D2 is proximal to and in parallel with each of the parting line grooves D1.
In FIG. 6(e), a light receiving side transparent electrode layer 6 is deposited so as to cover the parted semiconductor photoelectric conversion layer 5 and the parting line grooves D2. This light receiving side transparent electrode layer 6 can be formed by depositing an ITO (indium tin oxide) layer inside an electron-beam vapor deposition device.
Finally, in FIG. 6(f), the substrate taken out of the electron-beam vapor deposition device is set on the X-Y table, and a plurality of parting line grooves D3 are formed by use of the YAG laser beam LB3 incident from the light receiving side transparent electrode layer 6 side to part the light receiving side transparent electrode layer 6 into a plurality of regions. In this case, although the light receiving side electrode layer 6 is transparent, since the semiconductor photoelectric conversion layer 5 being apt to absorb a laser light is present under the light receiving side transparent electrode layer, heat generated inside the semiconductor photoelectric conversion layer 5 can also be used to perform processing of parting the light receiving side transparent electrode layer 6 with relative ease. In such a manner, the integrated-type thin film photoelectric converter is completed.    Patent Document 1: Japanese Patent Application Laid-Open No. H10-79522